Percentage of calcium in skeleton vs soft tissue, total body content
1000g, 99% in skeleton, 1% in soft tissue including extracellular fluid
Phosphorous distribution in body
600g: 85% in skeleton, 15% in soft tissue. Calcium/phosphorous=10/6
Magnesium body distribution
25g, 65% in skeleton, 35% in tissue
Extracellular fluid and Ca
500x more calcium in ECF than cytosol. Osmotic pressure pushes Ca to enter cell but ATP pumps keep gradient. Also Ca concentrated in mitochondria and ER as intercellular reservoir
Total concentration of blood in the blood
8.5-10.5mg/dl, about half ionized and the rest stays in blood bound to albumin and Igs. Minor amounts complexed with phosphate, citrate, sulfate, etc.
Calcium uptake into body
Dairy and wheat in diet provide about 1000mg, which goes to the gut, 300 of this gets taken into the gut, and 175 of this goes into ECF, 125 gets excreted in addition to the 700 that didn't get absorbed by the gut and 825mg gets excreted in feces. Pool of ECT is 900mg, 500 of which goes to the bone and 500 gets resorbed in equilibrium. 10,000mg passes through the kidney, of which 175 gets excreted in urine and 9825 goes back into ECM.
PTH and calcium and phosphorous
1. acts on bone by increasing osteoclasts (by progenitor differentiation). 2. acts on kidney to produce active form of vitamin D (1,25hydroxyD)-indirectly acts on absorption of calcium in gut by increased vitD metabolism. 3. reabsorption of calcium in the kidney.4. decreases phosphate resorption
Hypercalcemic, single chain peptide hormone, affected by low Ca levels, secreted by chief cells of PT glands. 84AA secreted. Terminal end has the bio action.
Structure of PTH
Amino end: Pre 25AA, Pro 6AA, active section is 34AA, carboxyl end 50AA. 59-70: membrane binding, 65-71: signal seq cleavage, 71-83 secretion.
Normal plasma calcium: PTH secretion
8.5-10.5 mg/dl Ca corresponds to 1.25-0.25 ng/kg/min PTH.
Amino terminal of secreted PTH
binds to receptors on bone and renal tubule
Two signal trnasduction pathways for PTH
1. binds to Gs protein-->adenylate cyclase-->cAMP which acts as a second messenger and activates PKA. 2. binds to Gq-->phospholipase C, activates PIP2 which splits into IP3 (releases Ca from ER) and DiAG-->PKC.
PTH binding to tubulin cell in the nephron
Binds to specific receptor on contraluminal membrane-->adenylyl cyclase actavted which catalyzes ATP-->cAMP which combines with kinase to go to PKA and phosphorylates C which phosphorylates the transporter for HPO4- and Na+, are charged and need to cross lipid membranes by specific transporters on lumina side, inhibited when phosphorylated. PTH causes hyperphosphaturia.
Control of PTH secretion through CaSR (calcium sensing receptor)
Blood ionized Ca directly suppresses PTH secretion by activating CaSR ( a dimer). Ca sensing receptor (CaSR) recognoizes Ca through G-protein coupled receptor expressed in the PT (also kidney). Ca ions in ECF are ligand for extracellular first messenger. Increased Ca causes CaSR activation-->intracellular signal-->decreased PTH.
Induced by suckling cuases prolactin release, first found in lactating women. 140AA. Also produced in tumor cells (50% of breast tumors, 17% of non-bone metastases, 92% of bone metastases)
PTH biological effect
End: increase serum Ca and increase urine P, increase osteoclast progenitor differentiation to release Ca from bone. Progenitors do not contain PTH receptors, receptors are on osteoblasts and stroma cells, produce proteins that affect progenitor differentiation.
PTH vs PTHrP
Most important: PTH regulated by Ca and PTHrP not affected by serum calcium. Diff genes, common receptor, similar biological and cell effects, PTH expression only in PT gland, PTHrP in mammary glands, normally a local regulator and not found in circulation, otherwise tumors. Both to do with bone development.
Osteolysis and PTHrP
Self-propelling process for bone resorption. PTHrP induces osteoblast expression of osteoblast derived factor (ODF) on the OCL progenitor to differentiate into osteoclasts, which then release TGFbeta which make tumor cells produce more PTHrP.
Osteoclast signalling for osteolysis by tumor cells
Prostaglandins, growth factors, PTHrP, cytokines, proteolytic enzymes
Peptide hormone produced by the thyroid. Net decrease in plasma Ca. Inhibits bone resorption, increases Ca excretion and decreases Ca resorption from kidney. Only active as a monomer (not dimer with disulfide bonds). Secreted most with hypercalcemia.
Naturally in animals, produced endogenously in skin from UV rays, present in nutrients with animal origin (liver, fish/oil, fortified dairy),m biologically inert and inactive, active only after metabolized in body (liver and kidney), 27 carbones, main fortification in Israel.
Formed by irradiation of ergocalciferol, found in plants (fingi important source), composed from 28carbones, extra methyl group at C24 and extra double bond. Less activity than D3 (esp in elderly), cheaper than D3, main fortification in US
What is the only active form of vitamin D?
Metabolism of 25-Hydroxyvitamin D3
In the kidney. If high PTH, low Ca, low Pi, hydroxylation at C1 by 1-hydroxylase to 1,25 dihydroxyvitamin D3. If low PTH, high Ca, high Pi, 24 hydroxylase transfers to 24,25DihydroxyvitaminD3, 1st step in degradaation of cholesterol, steroids, and vit D.
Formed in the liver, hydroxylated from D3 or D2 in position 25. Not an active form of vitamin D. Main vitD metabolite in circulation. Measured for vitamin D in labs.
1,25 dihydroxyvitgamin D3
The active metabolite of vitamin D. Induces 24 hydroxylase which will destroy it as negativer feedback. Hormonal form of vitD. Increases calcium absorption under strict regulation. Induced by low calcium, high PTH.
Relationship between calcium regulation and products of vitamin D
As calcium decreases, 1,25dihydroxyvitamin D3 increases and 24,25(OH)2H3 decreases. The turning point seems to be at serum calcium of about 9mg/d (normal), where as calcium increases, 1,25 drops off and 24,25 shoots up.
Serum phosphate and products of vitamin D
Normal Pi is 3-4 where 1,25 is most active and 24,25 is near zero. 24,25 starts increasing at 7 Pi and 1,25 has reached zero by 9mg%.
Estrogen and vitamin D
Increased estrogen levels increase 1,25 activity and decrease 24, 25 activity. In study with incubatino of 25, early peak of 24 and later 1,25 peak at higher estrogen levels. Consequence: postmenopausal women have decreased vitD active.
Testosterone and vitamin D
As testosterone increases, dose dependent increase in 1,25 production. In older age men have increased osteoporosin. But estrogen is decreaed in younger age in women than testosterone in men.
Lag period for vit D products
longer for inactive vitD, highest calcium absorption for 1,25. All level off after 24 hours.
Vitamin D effects cascade
1,25 increases Ca absorption in the gut which decreases PTH. PTH increases 1,25 and Ca, Ca inhibits 1,25 formation. 1,25 increases VDR which decreases PTH. High PTH increases bone resorption.
Mode of action of vitamin D
Binds a nuclear receptor. Gene activation: enhances transcription by RNA polymerase. Gene repression: blocks transcription. Diffuse through PM after release by binding protein, binds to VDR (vitamin D receptor) complex: ligand and 1,25, VDR binds to RXR to bind to DNA and VDRE (response element)
Forms of vitamin D and affinity to VDR
24 and 25 have about 2,000x less affinity than 1,25
Anticarcinoogenic effect of vitamin D
Downregulates MYC (proliferation factor), limits proliferation of oncogenes. Used for breast and colon cancer.
UVB exposure and yield of vitamin D exposure
White skin: yield of vitamin D 20 minutes, very dark skin: 120 minutes. Problem for example: Pakistni immigrants to England.
Excess of vitamin D and hypercalcemia
Most vitD is intert and 1,25 is highly regulated. But limit of consumption is 250nM of 25 (up to 32,000U/day of vitD3). In any case, serum calcium stays the same. No excess excretion in urine.
Vitamin D deficiency
0-25nM/L (<10ng/mL) deficiency, can lead to rickets or osteomalacia. 25-75nM/L (10-30ng/mL)insufficient, leads to osteoporosis, 75-250nM/L (30-100ng/mL)-sufficient, normal.
1,25 Vitamin D and HL60 leukemic cells
1. Inhibition 2. Induction of differentiaton into monocyte-like normal cells
Ketoconazole and vitamin D
Deficiencies in vitamin D consumption
Disturbance in sun exposure, malnutrition, malabsorption, chronic diseases. Diet is usually poor, Crohn's and Celiac diseases, bile duct block
Disturbance of first step of vitamin D metabolism (-->25OHD in liver)
Phernobarbitol, diphenylhydantoin, Biliary cirrhosis, disturbances of normal liver activity.
Disturbances in vitamin D metabolism in hydroxylation of C1
Kidney problems, Hypoparathyroidism, pseudohypoparathyroidism, uremia (damages kidney), type I vitamin D resistance (genetic disease, mutation in 1-hydroxylase, won't respond to vitD)
Disturbances in vitamin D metabolism in the 1,25 receptor
Postmenopausal decrease in estrogen, type II vitamin D resistance (problem in binding to VDR to express activity, symptoms same as vitamin D deficiency-Ricketts in babies, etc, but treatment with vitamin D did nothing, continuous high livels of 1,25 but no absorption into cells. Need to change activity of receptor, more complicated. Tx: direct calcium and phosphate by infusion).
Pathophysiology of renal bone disease
Two problems: 1. Decreased 1,25 production, less Ca absorption, leads to osteomalacia. 2. Decreased GFR-->increased phosphate (which by itself reduces 1,25 activity)-->decreased serum calcium-->increased PTH-->increased bone resoprtion-->osteitis fibrosa
Metabolic pathway of vitamin D and 1alphacalcidol
Used for uremic patients. If you give them 1,25 straight out, can jump to hypercalcemia and need to be monitored. Normally, D3-->25OHD3 in liver and in kidney-->1,25. But if there is kidney disease can use alternate previously hydroxylated alphacalcidiol which uses hydroxylase in the liver to go straight to 1,25. Also directly decreases PTH.
Physiological Roles of Calcium
Membrane stabilization, intracellular signal transduction, transmembrane ion exchange regulation (hypocalcemia-neuromuscular excitability, hormone and neurotransmitter release), cofactor in enzymatic processes (coagulation cascade, digestive enzymes), bone mineralization)
Distribution and function of calcium and phosphate
Ca much lower intracellularly than in serum and phosphate same or higher intracellularly.
Distribution of calcium in human plasma
40% protein-bound (nondiffusible), 10% complexed and 50% free and ionized (only physiological role)
Clinical calcium measurement
Routine: total serum calcium (no biologically-relevant ionized), subject to interference
In vivo factors that may influence calcium measurement
Intravascular calcium is more concetrated, tourniquet and venous occlusion (increased total), changes is posture (recumbency decreases), exercise (increases free), hyperventilation (decreases), meals (increase), increased albumin (increased total), myeloma (increases total calcium), high pH decreases free Ca, heparin (decreases), pyruvate, beta hydroxybutyrate and other ions decreases free calcium, in vitro factors.
Overview of calcium homeostasis
Decrease of calcium in ECF from calcium receptor will increase PTH-->stimulate 1,25, increase Ca absorption in kidney, stimulates bone resorption. 1,25 will increase absorption from intestine. Opposite when Ca increases ECF.
Pathogenesis of hypercalcemia
Results from imbalance between entry and removal of calcium in and out of extracellular compartment (too much PTH or PTHlike activity, increaed Ca threshold (decreased sensor activity-from mutation, need more Ca to reach same level of activity of normal receptor), exaggerated intestinal Ca absorption (excess of vitD or metabolites-mostly, excessive calcium intake-rare, exaggerated bone resorption (main cause of hypercalcemia in patients with cancer/bone metastasis), inability of Ca to be excreted by kidney (malfunction of kidneys, otherwise compensate for intestinal or bone resorption but overwhelmed, causes polydypsia, dehydration)
Differential diagnosis of hypercalcemia
PTH-like activity, low serum Pi (because PTH causes internalization of phosphorous channel in proximal tubules in kidneys, increased excretion). non-PTH-like hypercalcemia, increased serum Pi (because PTH suppressed, decreased phosphorous excretion. Renal function is normal because that in itself can cause increase in serum Pi).
Renal manifestations/complications of severe hypercalcemia
Polyuria/polydipsia-due to influence of calcium sensor receptor on ADH system, dehydration, renal failure, nephrolithiasis/nephrocalcinosis-precipitates in kidneys and ureter/extreme condition where calcium precipitates in renal matrix.
GI manifestations/complications of severe hypercalcemia
Nausea/vomiting/anorexia, abdominal pain, constipation, pancreatitis. From inhibitory effect of calcium on neural system.
Manifestations/complications of severe hypercalcemia
Renal, GI, Cardiac (bradyarrthmias, heart block), CNS (fatigue, depressed mentation, coma, death).